A Field-Effect Transistor (FET) is a type of transistor that relies on an electric field to control conduction of charge carriers through a conduction channel, for either negatively charged electrons in n-channel devices or positively charged holes in p-channel devices. One common type of field-effect transistor is a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), with an n-channel device called an nMOSFET (or simply NMOS) and a p-channel device called a pMOSFET (or simply pMOS). Complementary Metal-Oxide-Semiconductor (CMOS) is a class of integrated circuits that uses combinations of pMOSFETs and nMOSFETs to achieve logic functions.
In a MOSFET, the applied voltage on the gate electrode controls electrical current flow between a source terminal and a drain terminal via electrostatic control of the height of the conduction band edge energy within the channel (which extends between the source and the drain) in n-channel devices, or the valence band edge in p-channel devices. The “threshold” voltage is the rough point of switching the transistor between the active or ON state and the de-active or OFF state. For instance, in an nMOSFET, when the gate voltage exceeds the threshold voltage, electrons from the source (and possibly also the drain) enter the conducting channel. Current is conducted through the channel when a voltage is applied between the source and the drain. In this scenario, the nMOSFET may be said to be in the ON state. For gate voltages below the threshold value, the channel is lightly populated with electrons, and a very small current, referred to as the leakage current or subthreshold leakage current, can flow between the source and the drain. In this scenario, the nMOSFET may be said to be in the OFF state. In this manner, the transistor switches between an ON state and an OFF state.
However, switching between the ON and OFF states is not entirely abrupt. At best, the current can be reduced only by one order of magnitude for every 2.3 kBT/q (natural log of 10 times Boltzmann's constant (kB) times temperature (T) in degrees Kelvin all divided by the magnitude of the charge of an electron (q)) which is 60 mV at approximately room temperature (300 degrees Kelvin) when the transistor is switched to the OFF state. This limit is a result of “thermionic emission” of energetic charge carriers from the high energy tail of the carrier energy distribution in the source into the channel. The thermionically-emitted charge carriers represent a critical leakage path (leakage current) for MOSFETs in the OFF state. Thermionic emission is a basic physical mechanism of transport in a MOSFET and cannot be eliminated by changing device materials, the device geometry or the overall size of the device.
In attempting to minimize power consumption in CMOS logic employing MOSFETs, where the transistors are only switching for a very small fraction of the time on average and otherwise remain in a steady-state condition, transistor ON-OFF current ratios of multiple orders of magnitude (multiple factors of ten) still must be achieved to control OFF-state power consumption. To achieve these ratios subject to the optimal 2.3 kBT/q per decade switching and to also provide enough ON-state current for sufficiently rapid switching, an approximate half a volt change in the gate voltage between the ON and OFF states is required. However, the energy consumed during switching varies as the square of the supply voltage. Thus, historically, as device density has increased in logic circuits, not only have device dimensions been reduced, but also supply voltages. However, the inability to further scale supply voltages for MOSFETs beyond the point discussed above represents a major determent to the continued improvement in the computational capabilities and energy efficiency of future logic circuits employing MOSFETs.
If, however, field effect transistors could be adapted to have greater gate control over the current in the channel, such that even charge carriers with enough energy to reach the portion of the conduction channel beneath the gate will nevertheless be reflected back to the source in the OFF state, the limits of thermionic emission could be overcome thereby allowing reduction of the supply voltage and power consumption.